The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Automatic transmissions use a fluid clutch known as a torque converter to transfer engine torque from the engine to the transmission. The torque converter operates through hydraulic force provided by pressurized fluid from the automatic transmission. The torque converter multiplies engine torque and directs it through the transmission.
A conventional torque converter includes a sealed chamber filled with hydraulic fluid. The chamber includes a pump (or impeller) driven by the engine, a turbine connected to an output shaft, and a stator that provides torque multiplication. As the impeller rotates, the centrifugal force pushes the pressurized fluid outward, causing the turbine to rotate. Fluid exiting the turbine strikes the stator. Blades of the stator act to reverse the radial direction of the fluid's motion so that the fluid is moving the same direction as the impeller when it reenters the impeller chambers. This reversal of direction greatly increases the efficiency of the impeller. The force of the fluid striking the stator blades also exerts torque on the turbine output shaft, providing additional torque multiplication equivalent to a higher numerical gear ratio.
A torque converter is said to “slip” when the impeller speed and the turbine speed are not equivalent. High slip rates reduce the efficiency of the torque converter and may generate excessive heat. Some converters incorporate a lockup mechanism such as a mechanical clutch that engages at cruising speeds to physically link the impeller with the turbine. The physical link causes the impeller and the turbine to rotate at the same or near the same speed, thereby reducing or eliminating slip. The clutch is applied and released via fluid supplied through a hollow shaft at the center axis of the rotating converter assembly.
Engaging the torque converter clutch is not desirable in all modes of vehicle operation. Lockup conditions prevent the torque converter from providing torque multiplication. Instances may occur, for example, when driving along the highway and the driver steps on the accelerator pedal to pass another vehicle (referred to below as a throttle tip-in). The vehicle is operating in a higher gear with low engine speed (i.e. less than 2000 rpm) and the torque converter clutch is locked. If the current speed is above the requisite speed to initiate a downshift, the engine will remain at the low speed and the lockup will prevent torque transfer that is sufficient to accelerate the vehicle.